This invention relates generally to the construction of timepieces, and more particularly, to the design and construction of an improved bridge (i.e. the complementary part that is typically fixed to the main plate to form the frame of a watch movement) for use therein.
As is well known in the art, such bridges provide several functions, some of which are to provide mounts onto which gears or driving wheels may be rotateably secured, or to provide apertures through which gear/wheel stems may be positioned, to assist in maintaining the positioning of components of stepping motors such as the stators and rotors thereof, to provide a guiding support for many of the gears in a typical gearing assembly, and to provide for the construction of a compact and secured module in the watch. As such (and as actually shown in FIG. 2 which is the subject matter of the present invention), the front (i.e. motor) side of a bridge usually has many grooves, flanges and guides within which the motor wheels, gears, or other structures are positioned or supported. In the past, watch designers have used both metal and plastic for such bridges.
A solid one-piece plastic bridge has been found to be satisfactory when used in simple watch designs, such as in a typical 3-hand movement (not a chronograph) watch, wherein only one stepping motor is needed. The reason that such a one-piece bridge has been found to be satisfactory is because such bridges are typically small, such as 10 mm wide, and the allowable tolerances are usually easy to stay within. That is, within this 10 mm surface area, all the molded grooves, flanges and guides must be aligned very precisely so that even at the outer edges of such a surface, the gears and/or wheels (for example) do not rub up against the sidewalls of the grooves within which they are positioned, and within such a small surface area, current molding technology can provide for precision (i.e. tolerances) on the order of 10 μm. With only one motor, such tolerances are acceptable.
However, as one starts to require larger bridges because of multiple motors, for example, the molding technology and the inherent characteristics of the plastic bridge material itself (bending, shrinkage, etc.) significantly increases the prior acceptable margin of error. That is, as the bridge surface becomes larger, precise dimensions and relative measurements cannot be maintained across the entire bridge surface. This becomes a significant problem when multiple stepping motors are used, such as in a chronograph watch, because now, the molding technology and inherent characteristics of the bridge material itself cannot maintain the aforementioned 10 μm margin of error across the entire bridge surface. In fact, it has been found that when multiple motors are used, increases in what was otherwise acceptable deviations across an entire bridge begin to increase unacceptably, for example upwards of up to 20 μm. As such, when strict tolerances and precise measurements across an entire bridge are needed, such as when there are multiple motors located relatively far apart in different locations in the watch casing, a satisfactory one-piece plastic bridge has been difficult to manufacture.
For this reason, the construction of an optimal one-piece bridge, at least in plastic which is more economical and lightweight than metal, has been elusive.
FIG. 1 illustrates an exemplary conventional one-piece bridge for use in a chronograph watch, showing both a frame assembly 3 and the aforementioned one-piece bridge 2.
One solution to the aforementioned problem is simply to provide a plurality of disconnected sectional bridges, i.e. physically separated bridge sections. At first glance, this would seem to provide a way to keep the tolerances precise over each particular section. However, such a multipart construction requires multiple molds and precision and timely construction techniques since each section would warrant separate placement in the watch, thereby leading to increased costs, manufacturing time, and likelihood of errors, all of which are undesirable. Hence the use of a one-piece bridge is significantly more beneficial.
Unfortunately, the prior art has yet to construct a bridge that both achieves the desired needs while simultaneously overcoming the drawbacks set forth above. Accordingly, further developments in the construction of timepieces, and bridge for use therein in particular, are needed.